1. Field of the Invention
The present invention concerns a technique for forming a color filter used for CCD cameras, or various kinds of indicating elements, for example, liquid crystal display elements or color sensors and. it relates to a method of manufacturing colored layers or a black matrixes. More specifically. it relates to a method of manufacturing a new color filter capable of forming a colored layer or a black matrix simply and conveniently and at high resolution.
2. Description of the Related Art
Method of manufacturing color filters known at present includes (1) a dying method, (2) a pigment dispersion method, (3) a printing method, (4) an ink jet method, (5) an electrodeposition method and (6) a micelle electrolysis method.
Among them, both of (1) the dying method and (2) the pigment dispersion method have attained a high degree of technical completion and have been used generally for solid state color image pick-up devices but they require patterning by way of the step of photolithography, therefore causing a problem in view of the number of steps and high cost.
On the other hand, the printing method (3) and the ink jet method (4) require no photolithography step. However, the printing method (3) is a process of printing and then curing a thermosetting resin in which a pigment is dispersed and poor in view of resolution power or the uniformity of the film thickness. The ink jet method (4) is a process of forming a certain ink receiving layer, applying hydrophilic/hydrophobic treatment and then blowing ink to a hydrophilic portion to obtain a color filter layer but it involves a problem in view of the resolution power and high possibility of mixing color in adjacent filter layers and positional accuracy.
Color filters are generally formed by each of the methods on the side of an ITO substrate opposed to a TFT substrate. However, if the filter in formed at that position, it is necessary to positionally align the TFT substrate and the color filter substrate, making it difficult for attaining a high accuracy and increasing the cost.
The electrodeposition method (5) comprises applying a high voltage of about 70 V an a previously patterned transparent electrode in a solution of an electrolyte in which a pigment in dispersed in a water soluble polymer, and forming an electrodeposition film to conduct electrodeposition coating and repeating the steps for three times to obtain an R.G.B color filter. Since this method requires patterning for a transparent electrode by photolithography, which is used as an electrode for electrodeposition, it involves a drawback that the shape of the pattern in restricted and cannot be used for TFT liquid crystals. Further, if a color filter can be formed integrally by electrodeposition on a pixel electrode of a TFT liquid crystal substrate, no additional patterning is required. However, an electrodeposition voltage is high and it is extremely difficult to conduct electrodeposition to transparent pixel electrodes in an active matrix circuit in the existent electrodeposition method, making electrodeposition utilizing the pixel electrode of TFT impossible. Furthermore, it cannot be utilized, for example, by a reason that a driving voltage is increased since the color filter layer is insulative.
Japanese Published Examined Patent Application No. Hei 2-59446 describes to form a multi-colored device by forming a plurality of transparent conductive layers made of ITO, etc. being insulated from each other on a substrate, and repeatedly forming colored layers based on polymeric layers electrodeposited selectively on each of the conductive layers and discloses the use of a solution containing a water soluble polymer and a pigment as an electrodeposition solution. Japanese Published Examined Patent Application No. Hei 3-45804 discloses a method of using a substrate in which a TFT and a pixel electrode connected with the TFT are formed, driving the TFT in a state where the pixel electrode, a water soluble polymer and a dye-containing electrodeposition solution are in contact with each other thereby electrodepositing a colored film on a pixel electrode. Further, it describes repeating curing and formation of electrodeposition film on every color and, furthermore, forming a transparent electrode on the electrodeposition film. Japanese Published Examined Patent Application No. Hei 5-49967 discloses a method of forming a plurality of electroconductive transparent electrodes for liquid crystal display disposed being insulated on a substrate and selectively forming colored films by optionally applying a voltage selectively to the electrode.
Further, Japanese Published Unexamined Patent Application No. Hei 5-5874. for example, proposes a technique of forming a color filter integrally on the side of a TFT substrate. However, an electrodeposition film manufactured by a usual electrodeposition technique is insulative and, when the color filter is manufactured on a pixel electrode for liquid crystal driving, it cannot be utilized since the driving voltage for liquid crystals is increased. Further, impurities such as a surface active agent and the like are contained in usual electrodeposition films, purity of color or permeability is deteriorated or it is necessary to protect a TFT circuit from alkali metals contained in the film. Further, direct electrodeposition to the pixel electrodes the utilizing usual TFT driving circuits is impossible and therefore a TFT of high voltage withstand in necessary. In view of the above, a method of integrally forming a color filter substrate and a TFT substrate can not be put to practical use so far.
The micelle electrolysis method (6) is a sort of an electrodeposition method, and a voltage required for electrodeposition is lover since oxidation/reduction of ferrocene used an a deposition material is utilized, so that a color filter can be formed integrally by electrodeposition on the TFT liquid crystal substrate. However, in the thin film formed by the micelle electrolysis method, since ferrocene or surface active agent indispensable for the forming step are taken in and mixed as impurities, the color filter thus formed has poor permeability and poor color purity and has high resistivity. Further, it takes a long time such as several tens of minutes for electrodeposition to worsen the production efficiency. In addition, the ferrocene compound as the essential ingredient for electrolysis is extremely expensive to provide a problem in view of the cost.
Further, a color filter cannot generally be used as a color filter layer alone but a portion between each of the color filters and the pixel elements has to be covered with a black matrix. Usually, photolithography is used for forming the black matrix, and this is a major factor of increasing the cost. Accordingly, a method of manufacturing a color filter of a high resolution power with high controllability, and not using the photolithography and thus with less number of steps has not yet been known at present when considering the R.G.B. layer and the black matrix together, which causes poor yield and increase the cost in the production of the color filter.
The present invention provides a manufacturing method capable of manufacturing a high performance thin film transistor-integrated type color filter for use in liquid crystal display element, capable of saving alignment between a color filter and a TFT substrate, with less number of steps, reduced at the cost, having a high aperture ratio, high resolution and high controllability, by integrally forming a color filter by an electrodeposition method on the side of a substrate formed with a thin film transistor (hereinafter properly referred to as TFT).
The inventors of the present invention have reconsidered the electrodeposition technique itself in view of the principle. Then, the inventors have noted on compounds used for the method of manufacturing a filter proposed previously by the inventors as the photoelectrodeposition technique, which are water soluble and greatly changes water solubility depending on oxidized, neutral and reduced states, and have found that the electrodeposition materials can be used suitably also for forming a color filter on a TFT substrate as in the present invention without causing the problems as described above, and have accomplished the present invention.
The change of pH aqueous solutions utilized here generally occurs by way of electrolysis of water. Therefore, the start point of the voltage required for electrodeposition is an oxidation/reduction potential of water and an electrodeposition film is formed at a voltage much lower than that required so far for electrodeposition.
The electroconductive electrodeposition film and the technique relevant thereto proposed by the present inventors are based on the above findings and the outline for the method of forming images in the present invention resides in integral formation of a conductive color filter layer on a transparent semiconductor pixel electrode having a TFT circuit selectively as three color filter layers while utilizing the TFT circuit as it is.
That is, a method of manufacturing a thin film transistor-integrated type color filter according to the present invention comprises a transparent substrate manufacturing step of arranging and forming a thin film transistor and a transparent conductive film on a transparent substrate thereby forming a substrate for use in liquid crystal display and a step of bringing the substrate for use in liquid crystal display into contact with an aqueous electrodeposition solution containing a colorant, supplying an electric current, electrochemically changing the concentration of hydrogen ions thereby depositing the colorant on a transparent conductive film of the substrate for use in liquid crystal display and forming a conductive colored electrodeposition film.
According to the present invention, since a color filter of full three colors is formed on a pixel electrode of a TFT substrate by forming a color filter layer of the first color, among three colors of red (R), green (G) and blue (B), by electrodeposition only in a predetermined region using a transparent substrate in which a thin film transistor and a transparent conductive film are arranged and formed by utilizing an active matrix circuit thereof and then repeating the procedures for each of the colors, the color filter is formed on every pixel by a simple and convenient method and since they are formed directly on the TFT substrate, no alignment is necessary when it is used for liquid crystal display.
Upon forming the electrodeposition film, it is preferred to adopt a step of coating a photoresist on the substrate for use in liquid crystal display to expose the transparent conductive film by etching only at the predetermined region by entering a light on the side not formed with the transparent electrode in view of the protection for the TFT Circuit. In this embodiment, only the exposed portion of the transparent conductive film is brought into contact with the electrodeposition film and the electrodeposition film can be formed only at the portion by driving a predetermined electrode, while other portions are protected by the resist. In this case, if a positive type black resist material is used, the portion forms a black matrix.
Further, as a similar protection unit, it is also possible to dispose a protection film on the thin film transistor as an insulation film and form a colored electrodeposition film only for a predetermined portion by driving a predetermined electrode of the exposed transparent conductive film with no provision of the protection layer.
Further the black matrix can also be formed by separately providing a step of forming an insulative black matrix after forming a color filter by any of the manufacturing methods described above.
When a color filter is formed by previously disposing the transparent photoresist as described above, it is also possible to apply a step of forming a black matrix by subsequently removing the photoresist, disposing an insulative protection film for insulation from both of the color filter and the thin film transistor and then forming a black matrix on the protection layer.
In the manufacturing method according to the present invention, when the thin film transistor and the transparent conductive film are arranged and formed, the electrode portion can be provided with the same function as the black matrix by making the electrode of a low reflectance material, in which the step of forming the black matrix is not necessary.
In the same manner, by forming the electrode with a low reflectance material upon arranging and forming the thin film transistor and the transparent conductive film, further disposing an insulation layer to the electrode and then forming a pixel electrode, the electrodes and the electrode line can be provided with the same function as the black matrix.
In the method of manufacturing the color filter according to the present invention, the electrodeposition material used for an electrodeposition solution for forming the electrodeposition film preferably contains an electrodepositing dye or an electrodepositing polymer having carboxyl groups and having a nature of changing the solubility thereby causing deposition in accordance with the concentration of hydrogen ions. In this ease, if the dye itself has the electrodepositing property, additional use of a colorant is not particularly necessary, except for the purpose of controlling the hue. However, when a colorless or monocolor electrodepositing polymer is used, it is necessary to use a colorant having a desired hue together.
The electrodepositing polymer is preferably an electrodepositing polymer which is a copolymer of monomers having hydrophobic groups and hydrophilic groups, and in which the ratio of the number of hydrophobic groups based on the total number of the hydrophobic groups and the hydrophilic groups is from 40% to 80%, or an electrodepositing polymer which is a copolymer of monomers having hydrophilic groups.
In view of the purpose of forming a conductive color filter layer, the manufacturing method according to the present invention may adopt, a method of adding and dispersing fine particles of a highly conductive colorant or a transparent conductive material in an electrodeposition solution, or a method of adding, mixing and dispersing a salt giving no undesired effects on a TFT circuit as a conductive material. More specifically, when a highly conductive colorant is used, it is preferred to increase the ratio of the conductive material by using the electrodepositing polymer at a ratio of 1.5 or less relative to 1 for the conductive colorant, based on the weight % concentration ratio of the electrodepositing polymer and the conductive colorant.
Another embodiment of such conductive color filter layer can be obtained by forming a conductive electrodeposition film by the above-described unit and then forming a protection layer on the electrodeposition film in a state containing water in the electrodeposition film, so that electric charges can move in the electrodeposition film.
According to the manufacturing method of the present invention, the electrodeposition voltage required for forming an electrodeposition film in the step of forming an electrodeposition film can be lowered to 5 V or lower.
Further, in the step of forming a transparent substrate, a liquid crystal display element with a high aperture ratio and of high brightness can be attained by using a metal material of low reflectivity such as two- or three-layered Cr for the electrode upon forming the thin film transistor, by which the aperture ratio can be increased to the maximum, with no requirement for the black matrix.